Method for simultaneous determination of nitrogen and oxygen isotope compositions of natural nitrate and nitrite

Abstract

A method for simultaneous determination of nitrogen and oxygen isotope compositions of natural nitrate and nitrite, which quantitatively converts natural nitrate and nitrite into an organic ester and a nitro-compounds respectively, and then nitrate and nitrite δ 18 O and δ 15 N are simultaneously determined by adopting a gas chromatography/pyrolysis/gas chromatography/isotope ratio mass spectrometry coupling technology (GC/Py/GC/IRMS). According to the method for simultaneously determining the nitrogen and oxygen isotope compositions of the natural nitrate salt and nitrite salt, the small amount of sample does not result in the loss, acquisition, exchange and fractionation of nitrogen and oxygen.

What is claimed is: 
     
       1. A system for simultaneous determination of nitrogen and oxygen isotope compositions in nitrate and nitrite, wherein the system comprises:
 a nitrate and nitrite enrichment unit, configured for enriching the nitrate and the nitrite in a sample to be detected in the form of a HNO 3  and a HNO 2  to form an enrichment solution; 
 a chemical conversion unit, interfaced to the nitrate and nitrite enrichment unit and configured for converting the HNO 3  to a RONO 2 , converting the HNO 2  to a RNO 2 , wherein the R is a C 1 -C 20  alkyl group with or without an aryl substituent of 6-12 carbons; 
 a first gas chromatography separation unit, connected with a chemical conversion unit and configured for separating the RONO 2  and the RNO 2 ; 
 a pyrolysis unit, connected to the first gas chromatography separation unit and configured for pyrolyzing the RONO 2  and the RNO 2  to obtain pyrolysis gases, respectively containing CO and N 2 ; 
 a second gas chromatography separation unit, connected with the pyrolysis unit and configured for separating the CO and the N 2  in each pyrolysis gas; and 
 an isotope ratio mass spectrometry test unit, interfaced with the second gas chromatography separation unit, configured for performing δ 18 O and δ 15 N isotope analysis on the CO and the N 2 . 
 
     
     
       2. The system according to  claim 1 , wherein the chromatographic column of the first gas chromatographic separation unit is a first chromatographic column which is a medium-polarity gas chromatographic column or a weak-polarity gas chromatographic column; and the first chromatographic column is an HP-5 chromatographic column filled with 5%-phenyl -methyl polysiloxane, or a DB-HeavyWAX chromatographic column filled with polyethylene glycol. 
     
     
       3. The system according to  claim 1 , wherein the chromatographic column of the second gas chromatographic separation unit is a second chromatographic column, the second chromatographic column is a molecular sieve type chromatographic column, and the second chromatographic column is an HP-Molesieve chromatographic column, or an HP-PLOT Q chromatographic column. 
     
     
       4. The system according to  claim 1 , wherein the nitrate salt and nitrite salt enrichment unit comprises:
 an anion exchange column, configured for enriching anions in the sample to be detected; and 
 a hydrochloric acid supply device, connected with the ion exchange column and configured for washing the anion exchange column to obtain the enrichment solution containing HNO 3  and HNO 2 , and wherein a filling resin of the anion exchange column is AG1-X8 anion exchange resin in chloride form, or 717 strong alkaline anion exchange resin in chloride form; 
 or wherein the nitrate salt and nitrite salt enrichment unit comprises: 
 a cation exchange column, configured for enriching anions in the sample to be detected to obtain the enrichment solution containing HNO 3  and HNO 2 , and wherein the filling resin of the cation exchange column is 732 strong acidic cation resin in the hydrogen form. 
 
     
     
       5. The system according to  claim 1 , wherein the chemical conversion unit comprises:
 a silver oxide supply device; 
 a first reaction device, the silver oxide supply device is configured for supplying silver oxide to the first reaction device, whereby the silver oxide reacts with the enrichment solution in the first reaction device to form a product system containing a silver salt; 
 a first solid-liquid separation device, configured for performing solid-liquid separation on the product system to obtain a solution containing silver nitrate and silver nitrite, and wherein the first solid-liquid separation device is a centrifuge device; 
 a drying device configured so that the solution containing silver nitrate and silver nitrite is dried in the drying device to obtain a dry solid containing silver nitrate and silver nitrite, and wherein the drying device is a freeze-drying device; 
 a dissolving device, configured for dissolving silver nitrate and silver nitrite in the dry solid in the dissolving device by using an organic solvent to obtain a suspension liquid; 
 second solid-liquid separation device, configured for performing solid-liquid separation on the suspension liquid to obtain an organic solution of silver nitrate and silver nitrite, and wherein the second solid-liquid separation device is a centrifuge device; 
 an R-X supply device, wherein X is halogen; 
 a second reaction device, the R-X supply device supplies R-X to the second reaction device, and the second reaction device is configured for reacting an organic solution separated by the second solid-liquid separation device with the R-X to convert the silver nitrate in the organic solution into RONO 2  and the silver nitrite into RNO 2 ; and 
 a third solid-liquid separation device, configured for performing solid-liquid separation on a reaction product in the second reaction device, and wherein the third solid-liquid separation device is a centrifuge device. 
 
     
     
       6. A method for simultaneous determination nitrogen and oxygen isotope compositions in nitrate and nitrite, wherein the method comprises:
 step S1, converting nitrate in a sample to be detected into RONO 2 , converting nitrite in the sample to be detected into RNO 2  to form a solution to be detected, wherein R is a C 1 -C 20  alkyl group with or without an aryl substituent of 6-12 carbons; and 
 step S2, determining RONO 2  and RNO 2  in the solution to be analysed by adopting a gas chromatography/pyrolysis/gas chromatography/isotope ratio mass spectrometry coupling technology to perform δ 18 O and δ 15 N isotope analysis. 
 
     
     
       7. The method according to  claim 6 , wherein the step S1 comprises:
 step S11, enriching anions in the sample to be detected by an ion exchange means to obtain an enrichment solution containing HNO 3  and HNO 2 ; 
 step S12, reacting anions in the enrichment solution with Ag 2 O to obtain a product system containing AgNO 3  and AgNO 2 ; 
 step S13, performing a first solid-liquid separation on the product system to obtain a solution containing silver nitrate and silver nitrite, and wherein the first solid-liquid separation is centrifugal separation; 
 step S14, drying the solution containing silver nitrate and silver nitrite to obtain a dry solid containing silver nitrate and silver nitrite, wherein the drying is freeze-drying; 
 step S15, dissolving silver nitrate and silver nitrite in the dry solid by using an organic solvent to obtain a suspension liquid, wherein the organic solvent is acetonitrile; 
 step S16, performing a second solid-liquid separation on the suspension liquid to remove insoluble substances to obtain an organic solution of silver nitrate and silver nitrite, and wherein the second solid-liquid separation is centrifugal separation; 
 step S17, reacting the organic solution with R-X to convert the silver nitrate in the organic solution into RONO 2 , and converting the silver nitrite into RNO 2  to obtain a conversion system, wherein X is a halogen; 
 step S18, performing a third solid-liquid separation on the conversion system to obtain a solution to be detected, and whereinpreferably the third solid-liquid separation is centrifugal separation. 
 
     
     
       8. The method according to  claim 7 , wherein the step S11 comprises:
 performing ion exchange on anions in the sample to be detected by adopting an anion exchange column to obtain an anion exchange column enriched with anions, wherein a filling resin of the anion exchange column is an AG1-X8 anion exchange resin, or 717 strong alkaline anion exchange resin in chloride form; and 
 replacing anions containing nitrate ions and nitrite ions in the anion exchange column by hydrochloric acid to obtain the enrichment solution containing HNO 3  and HNO 2 ; 
 or the step S11 comprises: 
 performing ion exchange on the cation in the sample to be detected by using a cation exchange column to obtain the enrichment solution containing HNO 3  and HNO 2 , wherein a filling resin of the cation exchange column is a 732 strong acidic cation resin in the hydrogen form. 
 
     
     
       9. The method according to  claim 7 , wherein in the step S17, a ratio of the mole number of R-X to the total mole number of silver nitrate and silver nitrite is more than 1:1. 
     
     
       10. The method according to  claim 6 , wherein in the step S2, the concentration of the solution to be detected is 0.5 mg/ml to 1.0 mg/ml. 
     
     
       11. The method according to  claim 6 , wherein the step S2 comprises:
 separating RONO 2  and RNO 2  in the solution to be detected by using a first gas chromatographic column to obtain separated a RONO 2  and a RNO 2 ; 
 sequentially pyrolyzing the RONO 2  and the RNO 2  to obtain pyrolysis gases containing CO and N 2 ; 
 respectively separating the CO and the N 2  in the pyrolysis gases by using a second gas chromatographic column; and 
 performing isotope ratio mass spectrometry analysis on the CO and the N 2  sequentially to obtain δ 18 O and δ 15 N isotope compositions of CO and the N 2 . 
 
     
     
       12. The method according to  claim 11 , wherein the first gas chromatographic column is a medium-polarity gas chromatographic column or a weak-polarity gas chromatographic column, and wherein the first gas chromatographic column is an HP-5 chromatographic column filled with 5%-phenyl -methylpolysiloxane, or a DB-HeavyWAX chromatographic column filled with polyethylene glycol. 
     
     
       13. The method according to  claim 11 , wherein the second gas chromatographic column is a molecular sieve type gas chromatographic column, and wherein the second gas chromatographic column is an HP-Molsieve chromatographic column or an HP-PLOT Q chromatographic column.